This invention relates to an equalizer which is useful particularly in a satellite communication system.
In a conventional satellite communication system, a transmission line or path is established through a satellite between earth stations located at different terrestrial sites. With the satellite communication system, one of the earth stations is operable as a transmitting side or end for sending an output signal to the satellite through the transmission path while the output signal is delivered through the transmission path to the earth station serving as a receiving side or end.
On board the satellite is an amplifier which is usually operable in a saturation state in order to raise the efficiency of electric power on board the satellite. Stated otherwise, such an amplifier is put into operation in a nonlinear state. In addition, distortion and noise are inescapably imposed on the output signal for some reason during transmission. As a result, the transmission line or path inevitably has a nonlinear characteristic due to nonlinear operation of the amplifier and the distortion and the noise.
Under the circumstances, equalization of the transmission path is indispensable with the conventional satellite communication system in order to equalize or remove the nonlinear characteristic of the transmission path and to reduce a bit error rate on the transmission path in question. Herein, it should be noted that the bit error rate is dependent on the distortion of the transmission path and on a signal to noise ratio of the output signal transmitted from the transmitting side.
Satisfactory or optimum equalization can not be accomplished when the transmission path is equalized only between the satellite and the receiving side by the use of an equalizer set up in the receiving side, as known in the art. In other words, the transmission path should also be equalized between the transmitting side and the satellite for the purpose of achieving the optimum equalization. This is because the transmitting side uses an amplifier operated in a substantial nonlinear state and, in addition, the distortion and the noise are also imposed on the output signal on the transmission line extending from the transmission side to the satellite.
In order to realize the optimum equalization, a conventional transmitting side is also equipped with an equalizer of a transversal filter type comprising a plurality of delay elements having a plurality of taps and a control signal generator for generating a plurality of gain control signals which define tap gains at the respective taps, as will later be described with reference to one of several figures of the accompanying drawing. Equalization is carried out by changing or controlling the tap gains of the equalizer in the transmitting side.
It is mentioned that a gain or electric power of the transmitting side is varied with the change of the tap gains. As a result, the signal to noise ratio is not kept constant on the receiving side because of the variation of the gain of the transmitting side. This means that the bit error rate fluctuates during equalization when the tap gains of the equalizer is controlled on the transmitting side. Accordingly, the optimum equalization can not be expected with the conventional satellite communication system even when the transmitting side is equipped with such an equalizer.
Alternatively, it is known in the art to connect an automatic gain control circuit to the equalizer so as to avoid such variation of the gain of the transmitting side. Use of the automatic gain control circuit results in transient production of the output signal having an excessive amplitude like a surge in an initial state, because gain control operation does not begin in the initial state. The production of the excessive amplitude brings about an obstruction to the other transmission lines.